1. Field of the Invention
The present invention relates generally to a photosensor, and more particularly to a photosensor which includes a PIN photodiode which is formed of a P-type semiconductor layer, an intrinsic semiconductor layer and an N type semiconductor layer arranged laterally on an insulating substrate.
2. Description of the Prior Art
There has been proposed a photosensor as shown in FIG. 1 which is formed of a P-type semiconductor layer b and an N-type semiconductor layer c separated with an appropriate interval on a transparent insulating layer a made of quartz or the like. An intrinsic (I) amorphous semiconductor layer d is deposited between the P-type semiconductor layer b and the N-type semiconductor layer c so as to provide a lateral PIN photodiode PD which serves as a light receiving element. FIG. 2 shows an equivalent circuit diagram of the PIN photodiode PD. The PIN photodiode PD comprises a photo detecting circuit with a MOS transistor Q serially connected therewith, as shown in FIG. 3. A symbol .phi. designates a switching pulse for switching the MOS transistor Q and the reference letter C.sub.H represents an equivalent reverse bias capacitance of the PIN photodiode PD.
The lateral PIN photodiode PD can be manufactured in a manufacturing process which is the process for a PIN photodiode of sandwich construction which is formed of a P.sub.- type semiconductor layer, an intrinsic (I) amorphous semiconductor layer and an N-type semiconductor layer which are formed by one layer on another layer. The invention also requires a photo mask pattern which is relatively simple. Thus, the lateral PIN photodiode PD of the invention allows the production costs to be reduced.
However, although the lateral PIN photodiode PD has an advantage of a simple manufacturing process and a relatively simple photo mask pattern as mentioned above, the reverse bias capacitance C.sub.H is small, which causes a narrow dynamic range and an undesirable signal holding characteristic.
Due to the difference in construction between an ordinary sandwich PIN photodiode and a lateral PIN photodiode, the reverse bias capacitance C.sub.H of the lateral PIN is always extremely small, e.g. 1/20 or less that of the sandwich PIN photodiode. This problem will be explained in detail with reference to FIG. 4.
FIG. 4 is a characteristic graph showing the output voltage of the photo detector circuit shown in FIG. 3 in a dark field, specifically, the change in the output voltage of the photo detector circuit when the transistor Q is turned off from an on-state by the switching pulse .phi. wherein the abcsissa represents time t.
As is apparent from FIG. 4, with the previously proposed photosensor which employs a lateral PIN photodiode as a light receiving element, if the switching pulse .phi. falls as shown by the solid line, the output voltage Vx which is at substantially the same level as a source voltage Vv will abruptly decrease to ground level. On the other hand, with a sandwich PIN photodiode of the same size as the lateral PIN photodiode, the output voltage Vx slowly decreases from the source voltage level to ground level as shown by dashed line. This difference results due to the differences in the reverse bias capacitances between the sandwich and lateral PIN photodiodes. Specifically, the lateral PIN photodiode has a reverse bias capacitance which is far smaller than that of the sandwich PIN photodiode.
If the reverse bias capacitance is small, only a small amount of photo current causes the terminal voltage of the reverse bias capacitance C.sub.H to be maximum. In other words, the reverse bias capacitance is saturated with a small amount of photo current, which causes the dynamic range to be very narrow. Further, the holding characteristic which determines how long a signal can be held becomes short, which is not a desirable characteristic for a photosensor.